AD8307 Frequency compensated

The AD8307 is a nice logarithmic amplifier with a lot of uses for amateur radio. My SWR Power meter is build around it. (LINK) The frequency response is not so nice. I came across an older article by W7ZOI and W7PUA and they did some really nice improvements on this. Even better these improvements are easy to carry out and it really makes you measuring tool a lot better. You can find a copy of the article here build an rf power meter

I spent some time to do some measurements on the circuit and comparing it with the standard AD8307 set-up. I build 2 small boards with the compensation build on it.


The 2 “sloping” lines are the “normal” AD8307 . The large error shows immediately. As long as you calibrate your AD8307 on the frequency you want to use it there is not much what can go wrong. If you want to use it as a wideband measuring tool it is highly advised to ad the components as advised by W7POI and W7PUA. The gain difference is about 1dB. By careful tweaking it might be better. In series with the 14dB attenuator you see a small resistor, parallel with a capacitor and with a series inductor. This it all it takes. The reason why the yellow line drops so quickly below 75 MHz has not been investigated.


Top side of the board:


Bottom side of the board: